The present invention relates to flame retardant organopolysiloxane-polycarbonate block copolymers having nuclear bound silicon atoms attached to certain aromatic imide or aromatic ester groups by carbon-silicon bonds.
Prior to the present invention, silicone-polycarbonate block copolymers, such as shown by Vaughn, U.S. Pat. No. 3,189,662, were made by initially forming a polydimethylsiloxane having terminal bisphenol groups. A silicone-polycarbonate block copolymer was produced by phosgenating a mixture of the bisphenol-terminated polydiorganosiloxane and additional bisphenol. Although the resulting silicone polycarbonate block copolymer was found to possess valuable film forming properties useful in a variety of applications, the silicon-oxygen arylene linkage between the silicone blocks and the polycarbonate block was often hydrolytically unstable in particular situations. Improved hydrolytic stability was achieved by phosgenating polydiorganosiloxane having terminal Si-C bonded propylphenylhydroxy groups with bisphenol, as shown by Vaughn, U.S. Pat. No. 3,419,634. Although the hydrolytic stability of the resulting silicone polycarbonate block copolymers was improved as a result of a silicon atom being directly attached to a propyl carbon, the silicon propyl linkage did not offer the degree of thermal oxidative stability as would be provided by a silicon aryl linkage.
The present invention is based on the discovery that hydrolytically stable silicone-polycarbonate block copolymers having nuclear bound silicon atoms attached to arylene radicals by carbon-silicon bonds can be made by phosgenating mixtures of dihydric phenols, as defined hereinafter, with hydroxyarylimide siloxanes having the formula ##STR1## or hydroxyaryl ester siloxanes having the formula ##STR2## or mixtures of such siloxanes of formulas 1 and 2, where R-R.sup.7 of formulas (1) and (2) are selected from the same or different C.sub.(1-14) hydrocarbon radicals or C.sub.(1-14) hydrocarbon radicals substituted with radicals inert during equilibration or condensation, R.sup.8 is selected from C.sub.(6-13) divalent arylene radicals and C.sub.(6-13) divalent arylene radicals substituted with radicals inert during equilibration or condensation and R.sup.9 is selected from C.sub.(6-20) divalent aromatic hydrocarbon radicals, or C.sub.(6-20) divalent aromatic hydrocarbon radicals substituted with radicals inert during equilibration or condensation and n is an integer equal to from 1 to 200 inclusive.
It has been found that silicone-polycarbonate block copolymers can be made by phosgenating mixtures of hydroxyaryl imide siloxanes of formula (1) or hydroxyaryl ester siloxanes of formula (2) or mixtures thereof with one or more C.sub.(6-20) aromatic dihydroxy compounds. The silicone block of the resulting silicone-polycarbonate block copolymers is attached to the polycarbonate block by a silicon-arylene linkage. The silicone-polycarbonate block copolymers of the present invention exhibit flame retardant properties, and improved hydrolytic and oxidative stability and enjoy all of the valuable characteristics of the aforementioned silicone-polycarbonate block copolymers of the prior art, such as semi-permeability, injection-moldability and use in forming high performance films and coatings.